1. Field of the Invention
The present invention relates to a detachable sample cell for a sidestream respiratory gas sampling system, and, in particular, to a detachable sample cell that is readily removable from and replaceable in a sidestream respiratory gas sampling system.
2. Description of the Related Art
It is well-known by those skilled in the art that gas analyzers of the non-dispersive infrared (NDIR) type operate on the principle that the concentration of specific gases can be determined by (a) directing infrared radiation through a gas sample, (b) filtering this infrared radiation to minimize the energy outside the band absorbed by a specific gas in the gas sample, (c) measuring the radiation impinging upon a detecting device after having passed through the gas sample, and (d) relating a measure of the infrared absorption of the gas to the concentration of one or more specific gases being monitored. Gases that may be measured exhibit increased absorption (and reduced transmittance) at specific wavelengths in the infrared spectrum such that the greater the gas concentration, the greater the absorption, and, conversely, the lower the transmittance of the infrared radiation.
NDIR gas analyzers are widely used in medical applications and are typically categorized into two different types: (1) “diverting” or “sidestream” gas sampling systems; and (2) a “non-diverting” or “mainstream” gas sampling systems. A mainstream gas sampling system includes a sample cell that is disposed along the main path of a breathing circuit through which a patient's respiratory gases flow. As a result, the patient's inspired and expired respiratory gases pass through a sample cell, which is also known as a “cuvette”. A gas sensing system, which includes the elements necessary for monitoring respiratory gases such as a radiation source and detector, are coupled to the sample cell to measure the constituents of gas passing through the sample cell. An example of such a conventional mainstream Gas Measurement System is shown in U.S. Pat. No. 4,914,720 issued to Knodle et al.
A sidestream type of gas sampling system transports a portion of sampled gases from the sampling site, which is typically a breathing circuit coupled to the patient's airway or directly at the patient's airway, through a sampling tube to the sample cell, where the constituents of the gas are measured by a gas sensing system. Gases are continuously aspirated from the sample site, through the sampling tube, and into the sample cell, which is located within a gas measurement instrument. Gases are commonly sampled at flow rates ranging from about 50 ml/min to about 250 ml/min. The optical and electronic components associated with the sample cell for measuring the gas passing therethrough are positioned in the monitor a distance away from the patient's airway or a respiratory circuit. Examples of conventional sidestream gas sampling systems are taught in U.S. Pat. Nos. 4,692,621 to Passaro et al.; 4,177,381 to McClatchie; 5,282,473 to Braig et al.; and 5,932,877 also issued to Braig et al.
Conventionally, the sampling ports used by sidestream gas sampling systems are located in a wall of the respiratory circuit or an airway adapter therefor. The location of the sampling port along a breathing circuit may range anywhere from an elbow connected to an endotracheal tube to a wye connector at the opposite end of a breathing circuit. For example, the sampling port may be placed on the ventilator side of an in-line filter or heat-moisture exchanger (HME). This results in a drier sampling tube but with the inherent risk of significant distortion of the capnographic waveform and lower end-tidal values.
It is also well known in the art to locate the sampling port on the patient side of the in-line filter. However, there is a possibility of an accumulation of condensate and/or patient secretions in this configuration for a sidestream sampling system. Condensation from a humidified sample gas, in combination with patient secretions, can block and contaminate the sampling tube, which may necessitate frequent replacement thereof. To protect the sample cell from condensate, it is known to make the sampling tube permeable to water vapor, for example by using dehumidifying tubing, such as NAFION.®. brand tubing. It is also known to provide a water trap positioned at some point along the length of the sampling tube, a water filter also positioned along the sampling tube, or any combination of the dehumidification tubing, water trap, and water filter. The effectiveness of water traps and water filters vary between manufacturers, but no water trap or water filter is immune to eventual clogging and distortion of the capnographic waveform, particularly if preventive maintenance is inadequate.
Additionally, sources of leaks external to the gas monitor, such as loose fittings, cracked or slit sampling tubes, cracked sample filters, and cracked airway adapters, along with sources of leaks internal to the monitor, such as partial disconnection, are known to cause significant artifact in the capnogram output by conventional sidestream gas sampling systems. Leaks and obstructions can occur at any of the numerous connection points and tubes within a sidestream gas sampling system. As it may be difficult or impossible to calibrate for such artifacts, leaks, and obstructions, the capnographic waveforms and end-tidal measurements that are generated by use of sidestream analyzers may provide values that are significantly different from the actual values, which may, in turn, pose a potential hazard to the patient.
While more recent sidestream gas sampling system designs employ sampling ports that are located in the center of the adapter and, thus, along the flow path therethrough rather than at a wall thereof and, therefore, are less likely to aspirate secretions within a patient's respiration, they are still susceptible to the problems outlined above.
These problems are further exacerbated by the fact that the sample cells of sidestream analyzers are reusable and nondisposable, with windows that are formed from sapphire or other expensive materials. Thus, over time, condensation and contamination are likely to build up within such sample cells, reducing their performance over time. While the reusable sample cells of some sidestream analyzers may be removed therefrom for cleaning, the cleaning process is often avoided due to the high costs associated with replacing such sample cells. As a result, following the cleaning of such a sample cell, the accuracy of measurements obtained therewith diminishes over time.
Currently, the use of sidestream gas monitoring requires that careful attention be paid to the physical setup both external and internal to the monitor, and that care be taken in interpreting the capnographic waveform.
Given these problems with sidestream capnography, it is desirable to provide a sidestream gas sampling system that (a) is less prone to both internal and external leaks and obstructions, (b) provides data that more accurately reflects the true capnographic waveform of a patient's respiration, (c) is more robust with respect to accumulation of condensate and patient secretions, and (d) facilitates an easy determination of problems and corrective actions at the point of care should any of the above-noted problems occur with the sampling tube and/or the sample cell.